Ever since I got my current boat, I've had idle issues on both engs (468's).
The engs will idle at 800 RPM in neutral, but when drives are put in gear, the RPMS drop to 500-600 RPMs and die after abt 10-20 seconds if I don't catch them.
After idling thru a No Wake zone for a few mins, the issue is even worse.

I've dipped/rebuilt the carbs, replaced power valves, played with the mixture screws, adj'ed fuel press and adj'ed float levels many times (while on the water) but issue is still present.

The carbs are the Holley 4010/4011 series and only have idle mixture adjustments for the front barrels.
The heads are Merlins with the big rectangle ports and they also have the matching Merlin intakes.

I'm pretty sure the fuel is falling out of suspension due to low charge velocity thru the runners at idle.

Assuming the root cause is slow charge velocity, would modern carbs (i.e. QF 800/850's) really help much?
Anything else I could try?

These old carbs work great everywhere except at idle.

What is the timing at idle ?

Increase the idle to 900rpms. The drives will still be fine.

Need to check the timing.

If I set the idle speed screw to 900 rpm on the trailer, both engs will idle too high once they have cleared their throats. It takes a while for the idle to drop back down.
The way it is now; I set them to 650-700 rpm on the trailer with the idle speed screw. After a good run, they idle at abt 800-900 for a bit then start dropping.

You need to set float levels with the boat in the water if you have not been doing that way. The angle of the engines changes.

Set idle rpm when warmed up and in the water also.

put a timing light on it drop in gear in water, see if timing is backing off and causing stall, depending on cam etc, locking the advance and setting about 32 deg will help idle quality

x 100 on getting ignition straightened out before carb work.

That said, those 4010 carbs never did too well, thus why Holley dumped them.Not much adjust ability (tune ability)

When setting up the carb, like all Holley's, it is very important to also set the secondary throttle blade's opening when closed at idle. They have an idle circuit too, but it's fixed since no idle mixture screws. Set blades so the transfer slots are square typically .020"-.030"

Thx for all the replies!!

I'm not able to get the boat on the water today but I did check a few things mentioned.

The timing results were surprising.
The port eng was found set to 35 degs and does not change between idle and ~3k rpm.
The starboard eng at idle was found at ~40 degs (jumps around a few degs) and seems to increase a few degs more at 3k rpm.
Does the MSD ign need to have a jumper installed/unplugged or something to check base timing?

I'm running a 6AL's, MSD Blaster coils and MSD distributors:.

[ATTACH=CONFIG]559150[/ATTACH]

Pulled both carbs.
The secondary transfer slots were not visible at all on either carb:

[ATTACH=CONFIG]559136[/ATTACH]

[ATTACH=CONFIG]559137[/ATTACH]


Transfer ports with throttle held partially open:

[ATTACH=CONFIG]559139[/ATTACH]


After adjustment of stop screw for secondary's:

[ATTACH=CONFIG]559140[/ATTACH]

When adjusting the transfer ports, I first backed off the primary and secondary idle screws, loosened all the throttle plate screws 1/4 turn and snapped the throttles shut a few times to center the butterflies and re-tightened the butterfly screws. They seem to be closing all the way without any binding. Next, I adj'ed the secondary idle screws to provide a square shape on the transfer slots (~.030" exposed).

The engs both idle well but the idle speed is too high now on both engs. When first started, they idle at 900-1000 rpm. When warm they are at 1100-1200 rpm.
This is with the throttles disconnected, idle speed screw backed off all the way, idle mixture screws at 1.5 turns out and throttle held closed by hand just to make sure.

Checked for vacuum leaks but none found. The only vacuum hoses are the PCV hoses. When they are pinched off, the idle speed drops some (~100 RPM) but not enough.

I did discover that when the PCV valve is pulled from the valve cover, there's a loud air rushing sound, like there's a significant vacuum in the crankcase. Looked at the other valve cover and it had a PCV valve (that can't be right!?) which connects to the bottom of the flame arrestor/air cleaner.
I pulled both PVC valves from one eng and placed palms over valve cover openings to check for an internal vacuum leak, but didn't get a vac or a press build-up.

In regards to the carbs; only thing I can think of is to re-adj the secondary idle screws so the transfer port is closer to .015"-.020" of exposed port.

Thoughts??

Many marine engine run an open elbow that looks like a PCV at the valve cover. This is connected to flame arrestor.

Some do run a PCV. A PCV has a spring and ball inside . A PCV is connected to large vacuum port at back of carburetor.


So, check those things that look like PCV's and see if an open elbow or in fact a PCV.

Going to .020" should be fine for that circuit to work .

Most set total to 34-36 on BBC with factory heads.

Your MSD has weights and springs (mechanical advance) under the distributor cap. See if the plate is welded or if the weight / spring system works.